1. Field of the Invention
The present invention relates to a cathode for an electron gun used in a cathode ray tube, and more particularly, to a cathode for an electron gun for increasing its life cycle under a high current density load by ensuring a steady diffusion path of reducing component served for generating free radical barium.
2. Description of the Prior Art
A cathode ray tube is a device for forming an image by excitation light emission of a fluorescent material of a screen by landing an electron emitted from an electron gun and accelerated by high voltage on the fluorescent material.
FIG. 8 is a general structural diagram of a cathode for an electron gun in a cathode ray tube.
In FIG. 8, the cathode comprises a heater 4 in a sleeve 2, a cap-formed base metal 6 composed of nickel Ni as a main component and a small amount of reducing component such as silicon Si and magnesium Mg on the upper side of sleeve 2, and an electron emitting layer 8 mainly composed of alkaline earth metal oxide containing at least barium on the cap-formed base metal 6.
In such a cathode, the metal oxide and the reducing component react to each other by heat from the heater to generate free radical barium, and thereafter thermion is emitted by using free radical barium.
An electron emission capacity of the cathode for the electron gun is depended on a supply amount of free radical barium contained in the metal oxide.
However, since the cathode ray tube has a tendency of high brightness and long-life cycle recently, a cathode which can supply free radical barium for a long time in high current density is required.
A cathode restraining free radical barium from evaporating by adding both of lanthanum La compound and magnesium compound Mg or La--Mg mixed compound to the electron emitting layer containing alkaline earth metal oxide is disclosed.
However, in the conventional cathode, an intermediate layer 10 is generated in a boundary between the base metal 6 and electron emitting layer 8 by reaction as shown in FIG. 9, and it results to shorten the life of the cathode under high current density load of 2.about.3 A/cm.sup.2.
The intermediate layer 10 is generated by reaction of barium oxide pyrolized from barium carbonate and silicon or magnesium. EQU BaO+Mg.fwdarw.Mg+Ba.uparw. [Reaction formula 1] EQU 4BaO+Si.fwdarw.Ba.sub.2 SiO.sub.4 +2Ba.uparw. [Reaction formula 2]
Free radical barium generated by the reaction formula 1 or 2 is served to emit electron. However, MgO or Ba.sub.2 SiO.sub.4 is additionally generated by the same reaction formulas to generate the intermediate layer 10 in the boundary between the base metal 6 and the electron emitting layer 8.
Such an intermediate layer 10 interferes the reaction for generating free radical barium requiring the reducing component by obstructing diffusion of the reducing component contained in the base metal 6, to shorten the life of the cathode. In addition, since the intermediate layer 10 has a high resistance, it limits the current density possible to emit the electron by interfering flow of the electron emitting current.
In another aspect, a cathode for an electron gun comprising a metal layer mainly composed of tungsten of which the reducing degree is same as or smaller than silicon or magnesium and larger than nickel between the base metal and the electron emitting layer, and the electron emitting layer containing rare earth metal oxide to decompose the compound generated from the reaction, and thereby enabling the reducing component in the metal layer to serve to generate free radical barium is disclosed in Japanese patent laid-open No. 91-257735.
However, the cathode described above is stable at the beginning but its life cycle is suddenly deteriorated with the lapse of time since the additional reaction compound is generated when free radical barium is generated.